1. Field of the Invention
The present invention relates to a spindle device utilizing an externally pressurized gas bearing assembly, which device is provided with a rotary encoder used in precision machines and precision inspection equipments.
2. Description of the Prior Art
A spindle device utilizing an externally pressurized gas bearing assembly is well known, in which a rotary encoder is mounted on a shaft, such as disclosed in the Japanese Laid-open Patent Publication No. 10-239102. The spindle device utilizing the externally pressurized gas bearing assembly such as disclosed in the previously mentioned patent publication makes use of, separate from the shaft, a disc mounting member, on which a pulse disc forming a component part of the rotary encoder is mounted and which is secured to an end face of the shaft by means of bolts so that the geometric center of the plaid pattern on the pulse disc may be aligned with the axis of the shaft during rotation of the latter.
The spindle device utilizing the externally pressurized gas bearing assembly is largely used in the field of manufacture and inspection of magnetic discs and optical discs. As is well recognized by those skilled in the art, in order to increase the recording density of those discs, it is necessary to record data signals in high density, hence, to increase the positioning precision of a magnetic head. For this reason, it often occurs that the high resolution rotary encoder is fitted to the spindle device utilizing the externally pressurized gas bearing assembly, so that the rotational position of the shaft can be precisely detected to correct the position of the magnetic head and the timing at which data are recorded and/or read.
For the high resolution rotary encoder, a reflection type optical rotary encoder may be utilized, in which a laser beam is used as a detecting light.
Since the high resolution rotary encoder provides a substantial number of output pulses per rotation, the frequency of the output pulses may be too high to be used as a feedback signal for controlling the rotation of a motor and, therefore, the conventional motor drive circuit is incapable of accommodating it.
In view of the foregoing, it is contemplated to build both of the high resolution rotary encoder for providing a correction signal and the conventional rotary encoder for providing the feedback signal in the spindle device utilizing the externally pressurized gas bearing assembly.
However, this contemplated construction requires the two rotary encoders to be fitted to one end of the shaft of the spindle device of the specified type and, therefore, they protrude a substantial distance axially outwardly from a bearing portion for that end of the shaft, where the rotary encoders are mounted, and the runout precision tends to be degraded when unbalance occurs as a result of mounting of the pulse disc of the rotary encoder.
While the center of the disc mounting member is not generally aligned with the geometric center of the plaid pattern of the pulse disc, alignment of the geometric center of the plaid pattern of the pulse disc with the axis of rotation of the shaft results in misalignment between the center of the disc mounting member and the axis of rotation of the shaft, eventually resulting in the unbalance which leads to an increased fluctuation of the shaft during rotation. Also, where in the reflection type optical rotary encoder, a sheet-like scale is bonded to a metallic disc to form the pulse disc, the pulse disc has a substantial mass as compared with that made of a plate glass and, therefore, if the center of gravity of the pulse disc does not align with the center of the plaid pattern of the scale, even the slightest misalignment tends to result in a considerable unbalance.
In the event that the pulse disc tilts as a result of a machining error, rotation of the shaft leads to change of the distance between a detecting head and the pulse disc, constituting a cause of the detection error.
Because of those reasons, the position of the detecting head requires a precise adjustment relative to the pulse disc. If dismantling is carried out once the detecting head has been adjusted, re-adjustment of the position of the detecting head is required, resulting in increase of work steps. Also, where a laser beam is used as a detecting means, an error tends to occur in the output under the influence of irregular flow of air resulting from rotation of the pulse disc.